What is the ENERGY STAR sizing rule?

ENERGY STAR recommends roughly 20 BTU/h of cooling per square foot of floor area for a room air conditioner. The base capacity is then adjusted up for sunny rooms, extra occupants and kitchens, and down for heavily shaded rooms, to match the room's real cooling load.

What adjustments does it apply?

It reduces capacity by 10 percent for a heavily shaded room and raises it by 10 percent for a very sunny one. It adds 600 BTU/h for each regular occupant beyond two people, and adds 4,000 BTU/h if the room is a kitchen, where appliances add significant heat.

Why not just buy the biggest unit I can?

An oversized air conditioner cools the air fast but cycles off before it removes humidity, leaving the room cold and clammy and wasting energy. Matching capacity to the room keeps run times long enough to dehumidify and run efficiently.

Does this work for mini-splits and portables?

Yes. The BTU figure is a cooling capacity that applies to window units, ductless mini-splits and portable units alike. Portable units lose some effective capacity through the exhaust hose, so round up to the next size when choosing a portable.

Does ceiling height matter?

The 20 BTU/h per square foot rule assumes a standard 8-foot ceiling. For notably higher ceilings, increase the area input proportionally or add roughly 10 percent per extra two feet of height, since you are cooling more air volume.

What is the Room BTU Calculator?

Use the ENERGY STAR room air conditioner sizing rule — floor area times 20 BTU/h with adjustments for sun exposure, occupancy and kitchen use — to recommend the cooling capacity in BTU/h for a single-room unit. Built for homeowners and HVAC techs. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Room BTU Calculator — Gera Tools

Name: Room BTU Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Picking the right size air conditioner is the difference between a comfortable, efficient room and a cold, clammy one that short-cycles. This calculator uses the ENERGY STAR room air conditioner sizing method to turn your room’s size and conditions into a recommended cooling capacity in BTU per hour.

How it works

The ENERGY STAR rule starts from floor area and adjusts for the things that change a room’s real cooling load:

base       = floor area (ft²) × 20 BTU/h
sun         × 1.10 if very sunny, × 0.90 if heavily shaded
occupants  + 600 BTU/h for each regular occupant beyond two
kitchen    + 4,000 BTU/h if the room is a kitchen
recommended = base × sun + occupant add + kitchen add

Twenty BTU/h per square foot assumes a standard eight-foot ceiling. The adjustments capture solar gain, body heat from extra people, and appliance heat in kitchens, which all raise the load beyond the bare floor area.

Example and tips

A sunny 300-square-foot living room with three regular occupants needs about 300 × 20 × 1.10 = 6,600 BTU/h, plus 600 for the third person, for roughly 7,200 BTU/h — comfortably handled by an 8,000 BTU unit. For a kitchen, add the 4,000 BTU/h kitchen allowance. If you are buying a portable unit, round up one size because the exhaust hose costs you some effective capacity, and increase the area input for ceilings taller than eight feet.

Common room sizes and typical BTU results

Room size (ft²)	Condition	Approx. BTU/h	Suggested unit size
100–150	Shaded, 2 occupants	1,800–2,700	5,000 BTU
150–250	Average, 2 occupants	3,000–5,000	5,000–6,000 BTU
250–350	Average, 2 occupants	5,000–7,000	6,000–8,000 BTU
350–550	Sunny, 3 occupants	7,700–12,100	10,000–12,000 BTU
550–1,000	Average, living room	11,000–20,000	12,000–18,000 BTU

Unit sizes available at retail typically step in fixed increments: 5,000; 6,000; 8,000; 10,000; 12,000; 15,000; 18,000; and 24,000 BTU/h. Round up to the nearest available size.

Why oversizing is the more common mistake

The advice to avoid oversizing is counterintuitive — bigger seems safer — but an oversized unit creates real problems. Because it cools the air rapidly, it reaches the thermostat setpoint and shuts off before the evaporator coil has run long enough to pull significant moisture out of the air. The result is air that feels cold and clammy rather than comfortable, mould risk on walls and windowsills, and unit cycling that is hard on the compressor. A correctly sized unit runs longer cycles, dehumidifies more effectively, and often maintains a more even temperature.

Converting BTU to watts and kilowatts

Unit capacity in BTU/h converts to cooling watts as: watts = BTU/h ÷ 3.412. So an 8,000 BTU/h unit has approximately 2,345 watts of cooling capacity. This is the thermal output figure, not the electrical consumption: an air conditioner rated 8,000 BTU/h typically draws about 700–900 watts electrically, depending on its efficiency (EER or CEER rating). A higher EER means more cooling BTU per watt of electricity consumed.

Mini-split versus window unit versus portable

Window units are the most efficient and least expensive, but require a window that accommodates them and may not suit rental properties or historic buildings.

Mini-splits (ductless) are more efficient than window units, quieter, allow multiple zones, and require no window opening — but installation requires refrigerant lines through the wall and professional commissioning, adding to upfront cost.

Portables require no installation and move between rooms, but lose effective capacity through the hot exhaust hose. A unit rated at 8,000 BTU/h may deliver the equivalent of only 5,000–6,000 BTU/h of effective room cooling. When sizing a portable, round up by roughly one unit increment from the calculator’s result.